I was shown this in a client’s Webmaster Tools earlier this week:
We had carried out a number of 301 redirects on some of their pages, as for reasons known only to the original developer, a lot of pages had been created as sub-domains, which was causing duplicate content and indexing issues with Google.
What I wasn’t aware of, was that there isn’t any code in the site to auto-update the sitemap.xml file provided to Google Webmaster Tools. I hadn’t seen the error above before – clearly, Google is unhappy if too many of the URLs in your sitemap don’t match what it sees on the site. A lot of those URLs of co urse no longer exist (e.g. the sub-domains), so we have updated the sitemap using GSiteCrawler – it’s a bit techie, but it certainly does the job and can be scheduled to make regular updates with automatic FTP of the new sitemap.xml file.
So, if you’re making changes to your site, remember to update your sitemap.xml files!